1. Field of the Invention
The present invention relates to a recording medium such as an optical disc, a signal recording apparatus thereof, and a signal reproducing apparatus thereof.
2. Description of the Related Art
When digital data is highly densely recorded on a recording medium such as an optical disc, the digital data should be modulated with a large value of the minimum code inversion length Tmin so as to record as much data in a limited bit length as possible.
To do that, in recent years, as a modulation system with a large value of the minimum code inversion length Tmin, 4-9 modulation system has been proposed. The 4-9 modulation system is described in for example Japanese Patent Application No. 5-76692. The 4-9 modulation system converts 4-bit data into 9-bit data. In the 4-9 modulation system, the minimum code inversion length Tmin is 4T (where T is a bit length) and the maximum code inversion length Tmax is 18T.
FIG. 9 is a schematic diagram showing the construction of a code conversion table for the 4-9 modulation system. In FIG. 9, Dn is a value of which an input binary data sequence is divided by every four bits and represented in hexadecimal (HEX) notation. T.sub.n is a binary bit pattern in which the input D.sub.n is converted into 9 bits and at least three "Os" are present between two bit "1s". Due to the relation between D.sub.n and D.sub.n+1, two or three T.sub.n patterns are provided. Depending on the value of D.sub.n+1, the pattern of T.sub.n is determined. When D.sub.n =5, T.sub.n is either "000000100" or "000010001". When D.sub.n+1 is 6, 7, 8, D, or F, T.sub.n is "000010001". Otherwise, T.sub.n is "000000100". When T.sub.n is "000000100" , depending on D.sub.n+1, D.sub.n+2 also corresponds to the table of case (1). As a practical example, when D is 5, 6, or 7, "000010001", "000000000", or "100001000" can be obtained, respectively.
When data is recorded on the recording medium, the resultant 4-9 modulation code is modulated corresponding to NRZI (Non-Return to Zero Inverted) technique.
On the other hand, when data is reproduced from the recording medium, a synchronous code should be regularly recorded on the recording medium so as to read data, byte by byte. The synchronous code is hereinafter referred to as SYNC. The SYNC is a predetermined pattern that is not present in data portion.
In the 4-9 modulation system or the like, the signal that has been modulated corresponding to the NRZI technique contains a DC component. To suppress the DC component, a DC cancel code (DCC) is added to the data portion.
FIG. 10 shows the construction of a sector of a conventional optical disc. FIG. 11 shows the construction of a data portion of FIG. 10. As shown in these drawings, each sector is composed of an address portion 1 and a DATA/ECC portion 2. The SYNCs 3 and the DCCs 4 are regularly placed in at least the DATA/ECC portion 2. The DATA/ECC portion 2 is composed of a plurality of blocks, each of which has a predetermined bit length.
FIG. 12 shows the relation of positions of the SYNC 3 and the DCC 4 of FIG. 11. As shown in FIG. 12, data is placed between the SYNC 3 and the DCC 4. The signal length of the SYNC 3 is 36 bits. The signal length of the DCC 4 is 9 bits.
Although the DCC 4 is an essential code for suppressing the DC component of the wave form of a signal recorded on the disc, the data recording area of the DATA/ECC portion 2 is correspondingly decreased, thereby obstructing the high dense recording of the recording medium.
In the sector format of the conventional optical disc or the like, since the synchronous code and the cancel code are dispersedly disposed, the redundant portion other than the data portion becomes large, thereby restricting the amount of data recorded on the recording medium.